An anesthesia circulating loop is used for delivering gases comprising oxygen and other gases optionally infused with an inhalational anesthetic agent to a patient under positive end-expiratory pressure and for providing ventilation control to the patient. Positive end-expiratory pressure is the pressure in the lungs of a patient, clinically referred to as “alveolar pressure”, at the end of expiration. As used herein, the term “ventilation” refers to a process of providing mechanical assistance to a patient for breathing. The anesthesia circulating loop comprises an expiratory section and an inspiratory section. The anesthesia circulating loop allows breathing gases to be forced into the patient to preclude apnea, which is a major effect of anesthesia. As used herein, “breathing gases” refer to gases inhaled by the patient from the anesthesia circulating loop and gases exhaled by the patient into the anesthesia circulating loop during breathing. Also, as used herein, “inspiratory section gases” refer to gases transported along the inspiratory section of the anesthesia circulating loop, which are inhaled or inspired by a patient from the inspiratory section of the anesthesia circulating loop and gases bypassing inhalation and transported directly into the expiratory section of the anesthesia circulating loop. Also, as used herein, “expiratory section gases” refer to gases that are exhaled or expired by the patient into the expiratory section of the anesthesia circulating loop and gases bypassing inhalation and transported directly into the expiratory section from the inspiratory section which are not exhaled by the patient, both of which are transported along the expiratory section of the anesthesia circulating loop.
Conventional anesthesia delivery systems are typically configured as bag-in-box systems, where a circulation blower provides a continuous flow of gases and a ventilation drive provides ventilation into a box, which moves a bag. The ventilation drive typically includes another blower or pressurized air. A conventional bag-in-box system, also referred to as a bellows system, with a single in-line blower is a complex mechanical system and introduces additional cost and disturbance in ventilation patterns. Moreover, conventional bag-in-box systems have a single connection for inspiratory section gases and expiratory section gases that preclude complete mixing of the inspiratory section gases and the expiratory section gases, especially in low breathing volumes. Other conventional anesthesia delivery systems use a cylinder-piston system similar to the bag-in-box system. In addition to the costs involved and disturbances in ventilation patterns, conventional cylinder-piston type anesthesia delivery systems are more prone to leakage of gases and pose a high risk of occurrence of a negative end-expiratory pressure, which is a subatmospheric pressure that develops at a patient's airway at the end of expiration.
In other conventional anesthesia delivery systems, a combination of a ventilation blower and a ventilation valve provides ventilation to a patient. In these systems, a positive end-expiratory pressure is maintained in a circulating flow of gases by a complex feedback control system based on a set of flow sensors and control of the ventilation valve and the ventilation blower, which results in oscillations or a substantially slow reaction to remove the oscillations. Moreover, in conventional systems, a substantially large volume of gases retained in a conventional reservoir is bypassed by the circulation flow. When ventilation starts, a large offset occurs due to a previously unmixed volume of gases.
A positive end-expiratory pressure of about 2 cm water (H2O) to about 10 cm H2O is required to be maintained in the lungs of a patient to keep the lungs open and to prevent the lungs from collapsing during or at the end of expiration, or to assist with lung inflation, that is, alveolar inflation, during the ventilation of the patient. The positive end-expiratory pressure helps to keep the lungs, that is, the alveoli, open and reduces pulmonary edema, that is, ingress of liquid from the capillaries into the alveoli. The pressure inside the lungs at the end of expiration is typically about 0 cm H2O, that is, atmospheric pressure. A conventional anesthesia delivery system comprises a circulating loop with a flow proportional valve in the expiratory section of the circulating loop for restricting the flow of breathing gases, thereby creating a back pressure upstream of the flow proportional valve. The back pressure results in the creation of a positive end-expiratory pressure in the lungs of the patient by restricting the flow of breathing gases upstream of the flow proportional valve. The positive end-expiratory pressure is typically preset at about 2 cm H2O to about 10 cm H2O. In the anesthesia art, a flow proportional valve that functions to create a positive end-expiratory pressure in an anesthesia circulating loop is referred to as a positive end-expiratory pressure valve. A positive end-expiratory pressure valve is used in a conventional anesthesia circulating loop to maintain a pressure of about 2 cm H2O to about 4 cm H2O above atmospheric pressure within the patient's lungs. The positive end-expiratory pressure valve is typically positioned on an expiratory section of the anesthesia circulating loop with the position of the positive end-expiratory pressure valve selected by a manufacturer of the anesthesia circulating loop. In a conventional system, an adjustable spring located within the positive end-expiratory pressure valve is used for regulating the positive end-expiratory pressure at about 2 cm H2O to about 10 cm H2O in the patient's lungs at the end of expiration. In another conventional system, the 2 cm H2O to 10 cm H2O positive end-expiratory pressure required to be maintained in the lungs is obtained by adjusting a knob extending out of a clear dome of the positive end-expiratory pressure valve. In another conventional system, the positive end-expiratory pressure is regulated by changing the tension on a spring located inside a device on the expiratory section of the anesthesia circulating loop. In another conventional system, a heat and moisture exchanger type insert fitted between an expiratory limb and an expiratory limb port function as a positive end-expiratory pressure valve.
Some conventional anesthesia delivery and ventilation systems have a ball bearing in the positive end-expiratory pressure valve that provides gravity-induced resistance to exhalation. This positive end-expiratory pressure valve has to be oriented perpendicular to a ground surface to work properly. The positive end-expiratory pressure valve is not adjustable. If a user wants to go from 2 cm H2O to 10 cm H2O, the user is required to use a different valve with a heavier ball bearing. If the positive end-expiratory pressure valve is inadvertently installed upside down, the anesthesia circulating loop will be completely blocked. Moreover, resistance to exhalation stays the same when switching from a ventilator mode to a bag mode. Other conventional anesthesia machines have positive end-expiratory pressure valves that are electrically controlled to deliver the amount of positive end-expiratory pressure that is dialed into ventilator controls. With electrically controlled positive end-expiratory pressure valves, the positive end-expiratory pressure returns to zero when the anesthesia machine is switched from the ventilator mode to the bag mode.
To obtain the desired 2 cm H2O to 10 cm H2O positive end-expiratory pressure, at the start of an expiration phase, a command pressure maintained by an exhalation valve is lowered either abruptly or gradually from a desired inspiration pressure to the desired positive end-expiratory pressure. The patient exhales in the expiration phase. In a conventional anesthesia delivery system, the system pressure undergoes a steep drop initially, and oscillates about the desired positive end-expiratory pressure at a typical frequency of the anesthesia delivery system until equilibrium is reached. The frequency and amplitude of the oscillation depends, for example, on compressibility and volume of a respiratory gas, tolerances of components of the ventilation system, and the patient's health condition. The amplitude and duration of the oscillation at the beginning of the expiration phase can be substantial.
Some conventional systems use only a single in-line blower to provide both a continuous flow of gases and ventilation. In other conventional systems, an inline blower and a ventilation blower are used to provide both a continuous flow of gases and ventilation. In both these systems, a proportional valve, for example, a positive end-expiratory pressure valve is used for controlling the continuous flow of gases and ventilation patterns at the same time. There are several problems associated with the use of a proportional valve, for example, a positive end-expiratory pressure valve, to create a positive end-expiratory pressure in an anesthesia circulating loop. A positive end-expiratory pressure valve installed in the expiratory section of the anesthesia circulating loop increases the resistance to flow of the gases exhaled by the patient, and increases the breathing effort of the patient especially in low ventilation volumes. Moreover, the positive end-expiratory pressure valve increases the complexity of a control algorithm for simultaneously controlling the continuous flow of gases and ventilation patterns. For example, when there is no breath pattern needed, the positive end-expiratory pressure valve is kept completely open and the in-line blower is run at the lowest pressure and flow rate required for the continuous flow of gases. During inspiration and expiration, both the positive end-expiratory pressure valve and power to the single in-line blower and/or the power to the in-line blower and the ventilation blower must be controlled simultaneously and continuously to produce the required continuous flow of gases and inspiration pattern. The simultaneous and continuous control of the positive end-expiratory pressure valve and the power to the in-line blower and the ventilation blower increases the risk of an interruption or a lower than an optimal volume of the continuous flow of breathing gases. Furthermore, the use of a positive end-expiratory pressure valve increases the risk of a delay in the switching time between breath phases, for example, from expiration to inspiration. For example, with the use of a positive end-expiratory pressure valve, the inspiration cycle is maintained by closing the positive end-expiratory pressure valve, either completely or partially. To switch from inspiration to expiration, the positive end-expiratory pressure valve is opened to the level where the required positive end-expiratory pressure is maintained in the anesthesia circulating loop. During assisted ventilation, where the breathing effort of the patient is used as a trigger to start an inspiration or expiration, the positive end-expiratory pressure valve is controlled accordingly, which introduces a corresponding delay in opening or closing of the positive end-expiratory pressure valve, which in turn, can increase the breathing effort of the patient. The delay in the operation of the positive end-expiratory pressure valve also increases the risk of pressure oscillations of breathing gases in the positive end-expiratory pressure value during expiration. Furthermore, it is difficult to accurately control the positive end-expiratory pressure by a positive end-expiratory pressure valve at a narrow preset range of, for example, about 2 cm H2O to 10 cm H2O for an extended period of time, due in part to the time required to adjust the positive end-expiratory pressure valve in response to changing physiological and breathing conditions of a patient.
Interconnections between sections, for example, the expiratory section, the inspiratory section, etc., of the anesthesia circulating loop are based on tube connections where one tube is inserted into another tube. The interconnections are generally airtight and can be easily removed and reconnected. However, in conventional systems where a turbine is used for a blower, for example, the circulation blower, the housing of the turbine has two tubes on both sides that can be attached using tubes to the rest of the circuit. The housing of the turbine is made up of two halves with the turbine in between the two halves. To sterilize the turbine, the halves are dismantled and the turbine is removed. After sterilization, the turbine is placed between the halves of the housing and is clamped such that the turbine can rotate freely while joints between the turbine and the halves of the housing are airtight. The type of joint used between the turbine and the halves of the housing is prone to leakage. As used herein, “leakage” refers to leakage of gases from the anesthesia circulating loop at points of interconnection between different sections of the anesthesia circulating loop and at points where a turbine of a blower is mounted in the anesthesia circulating loop for in-line blowers if any. There is a need for preventing leakage of gases from the anesthesia circulating loop.
Hence, there is a long felt need for a method and an anesthesia delivery and ventilation system for delivering inspiratory section gases optionally infused with an inhalational anesthetic agent to a patient and for controlling positive end-expiratory pressure and ventilation without the use of a proportional valve. Moreover, there is a need for an anesthesia delivery and ventilation system that ensures uniform mixing of expiratory section gases and fresh gases that constitute the inspiratory section gases and that allows the inspiratory section gases in the inspiratory section to reach a required composition. Furthermore, there is a need for an anesthesia delivery and ventilation system where continuous circulation of the expiratory section gases and flow of the inspiratory section gases are controlled independently. Furthermore, there is a need for an anesthesia delivery and ventilation system that is less prone to leakage of gases from the anesthesia delivery and ventilation system compared to conventional anesthesia delivery systems.